Coatings for articles used with molten metal

ABSTRACT

An improved multilayer coating for use on molten metal holding and transfer apparatus, the coating including a bound layer applied directly to the surface of molten metal holding and transfer apparatus, and a porous layer of ceramic material produced by co-deposition of a powder of said ceramic material and a powder of a suitable organic polymer material and, after the co-deposition, heating of said polymer material to thermally decompose the polymer material and form the porous layer. The bond layer preferably is formed of a metallic, intermetallic or composite particulate materials. The metal component may be in the metallic, intermetallic, oxide, clad or alloyed form consisting of any one or more of the metal components selected from the group of Mo, Ni, Al, Cr, Co, Y and W and may be in combination with yttria, alumina, zirconia, boron, carbon and have a particle size in the range of 5 to 250 m, typically 40 to 125 m. The bond layer preferably has a thickness of 5 to 300 m with a substantially uniform coating layer being provided over the surfaces to have the porous ceramic coat applied.

TECHNICAL FIELD

This invention relates to coatings for articles used in handling moltenmetal and in particular relates to articles used for transferring,stirring and holding molten metal.

BACKGROUND OF THE INVENTION

In many molten metal handling operations, articles used to handle moltenmetal are often provided with coatings to protect the surface of thearticles from the erosive and corrosive effects of the molten metal. Inparticular, metallic and ceramic coatings have been used for a manyyears to change the surface performance of the refractory materials incontact with metal troughs, launders, ladles, skimming tools and siphontubes. All of these articles are in contact with flowing molten metal,thus exposing the coatings to not only corrosive attack from the moltenmetal but also erosion from the metal drag across the surface of thecoating. The thermally insulating nature of the coating also preventstemperature loss of the molten metal.

One possible solution to the erosion problem is simply to provide athicker coating. Unfortunately thicker coatings are prone todelamination and typically have less strength than thin coatings due tomicro cracking or low cohesive bonding.

Another problem with thicker coatings arises from the thermal expansiondifference between the substrate and the coating. Stresses arising fromthese thermal expansion differences become more pronounced with thickercoatings as they go through temperature changes leading to spalling ofthe thicker coatings. Because of the delamination problems and thermalexpansion mismatches, the ability to effectively thermally insulate themetal articles, wear resistance and service life is adversely affectedand thicker ceramic coatings are not extensively used for metal transferand holding apparatus.

By referring to the coatings as “ceramic based” the term “ceramic” wasused in its art recognised sense as being inorganic, non-metallicmaterials processed or consolidated at higher temperature” (McGraw-HillEncyclopaedia of Science and Technology 1994). The classes of materialsgenerally considered to be ceramics include oxides, nitrides, borides,silicides and sulfides. Intermetallic compounds such as aluminates andberyllides are also considered as “ceramics” as are phosphides,antimonides and arsenides.

In PCT/AU00/00239 an improved die coating for use on the surface of amould or die component contacted by molten metal in low pressure orgravity die casting was disclosed. In that reference, the coatingincluded a porous layer of ceramic material produced by co-deposition,using a thermal spraying procedure, of a powder of the material and apowder of a suitable organic polymer material and, after theco-deposition, heating of the polymer material (in an oxidizingatmosphere) to cause its decomposition and removal.

That invention also provided a process for providing a die coating onsuch surface of a metal mould or die component wherein an initialcoating of organic polymer material and ceramic material was formed onthe surface by co-deposition of powders of the materials by a thermalspraying procedure, and the initial coating was heated so as to removethe polymer material and leave a porous coating of the ceramic material.In low pressure and gravity die casting, the molten metal does notcontinuously travel across the surface of the mould, and so the effectsof erosion and wear resistance are not considered to be a significantconsideration.

The known die coating technology typically involved the use of awater-based suspension of ceramic particles in a water-based binder,most commonly sodium or potassium silicate. Coating mixtures of thistype needed to be properly stored and mixed. The coating was applied tothe prepared surface of a die component using a pressurised air spraygun. For this, the die component was preheated, typically from about 150to 220° C., such that water was evaporated from the die surface,enabling the binder to polymerise and bond the ceramic particlestogether and to the die surface.

However in liquid metal transport and holding applications, there issignificantly more metal drag on the coating. Thus, thermal mismatchesand lamination flow effects, play a much greater role in the servicelife and wear resistance of the coating.

The applicants have found that coating compositions as disclosed in thisPCT patent application surprisingly can be extended beyond the use indies described in that invention to liquid metal transport and holdingarticles.

SUMMARY OF THE INVENTION

Accordingly, the invention provides in one form an improved multilayercoating for use on molten metal holding and transfer apparatus, thecoating including a bond layer applied directly to the surface of moltenmetal holding and transfer apparatus, and a porous layer of ceramicmaterial produced by co-deposition of a powder of said ceramic materialand a powder of a suitable organic polymer material and, after theco-deposition, heating of said polymer material to thermally decomposethe polymer material and form the porous layer.

The applicants have found that the application of a bond layer to thesurface of the molten metal and holding apparatus prior to theapplication of the porous layer of ceramic material, reduces the thermalexpansion mismatch between the porous ceramic coating and the metalsubstrate, the application of this layer greatly enhances the physicalbond strength of the porous ceramic layer.

During use of the coated molten metal holding and transfer apparatus,the thermal mismatch between the substrate and the ceramic layer canresult in fine cracks appearing which initially can go undetected. Thisgreatly exposes the metal substrate to oxidation and erosion. Theapplicants have found that by providing a bond layer, not only is thethermal expansion mismatch reduced, substrate damage caused by oxidationand corrosion is also substantially reduced.

The bond layer preferably is formed of a metallic, intermetallic orcomposite particulate materials. The bond layer is formed from aparticulate material applied to the surface of the metal surface of thetransport, stirring or holding apparatus. The bond coat layer can beapplied by a thermal spray process such as vacuum plasma spray (VPS),atmospheric plasma spray (APS), combustion flame spraying and hypervelocity oxyfuel (HVOF) spray processes.

The metal in the bond layer may be in the metallic, intermetallic,oxide, clad or alloyed form consisting of any one or more of the metalcomponents selected from the group of Mo, Ni, Al, Cr, Co, Y and W andmay be in combination with yttria, alumina, zirconia, boron, carbon andhave a particle size in the range of 5 to 250 μm, typically 40 to 125μm. The bond layer preferably has a thickness of 5 to 300 μm with asubstantially uniform coating layer being provided over the surfaces tohave the porous ceramic coat applied.

After the bond layer has been applied to the metal surface of thetransport, stirring or holding apparatus, a ceramic and polymer powderis deposited. This ceramic and polymer powder is then heated tothermally decompose the polymer powders to leave a porous ceramic layeron the bond layer.

The ceramic powder making up the porous layer may be selected from atleast one metal compound such as oxides, nitrides, carbides and borides,preferably from the group comprising alumina, titania, silica,stabilised or partially stabilised zirconia, silicon nitride, siliconcarbide, and tungsten carbide.

Alternatively, the ceramic powder may be at least one mineral compoundselected from the group of clay minerals, hard rock ore and heavymineral sands such as those of ilmenite, rutile and/or zircon.

The organic polymer powder may be formed from a thermoplastic material,such as polystyrene, styrene-acrylonitrile, polymethacrylates,polyesters, polyamides, polyamide-imides and PTFE.

Preferably the ceramic and polymer powders are of relatively narrow sizespectrum and preferably in the range 20 μm-400 μm.

The ceramic and polymer particles which are used to form the porousceramic layer are of particle sizes not more than about 300 μm and notless than about 5 μm.

The porous coating may have a thickness of from about 50 to 600 μm and aporosity of up to 70% depending on its application.

More preferably the porous coating has a thickness of from about 100 toabout 400 μm. The insulating properties of the coating are a function ofthe coating thickness, the thermal conductivity of the ceramic as wellas the porosity of the coating.

The invention provides a process of providing a coating on the surfaceof an article that comes into contact with molten metal, wherein aninitial coating is applied to the surface of the article and a ceramicinsulating layer of an organic polymer material and ceramic material isformed on the surface by co-deposition of powders of the materials andthe coating is heated preferably to a temperature to decompose andremove the polymer material and leave a porous layer of the ceramicmaterial. This temperature is above the thermal decompositiontemperature of the polymer and up to 550° C. As the articles to becoated are metal, typically mild steel or cast iron, it is desirable toavoid temperatures above 600° C., as such elevated temperatures have aneffect on the tempering, microstructure and properties of the metalcomponents. In fact, above 900° C., the steel dies undergo an austeniticphase transformation which changes hardness and causes distortion of themetal components.

In order to produce a very smooth surface finish, an outermost layer offine ceramic material without polymer can be applied. This isparticularly useful where the coating is more porous.

In an alternative form, the invention provides an improved coating foruse on metal articles that are in contact with molten metals. Theimproved coating including a bond layer, a porous layer of ceramicmaterial produced by co-deposition of a powder of said ceramic materialand a powder of a suitable organic polymer material and, after theco-deposition, heating preferably to a temperature of up to 550° C. ofsaid polymer material to cause its removal.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described by reference to the followingnon-limiting example.

To reduce the thermal expansion mismatch between metal article and thecoating, a bond layer such as that described below was applied betweenthe coating and the metal surface of the transport and holdingapparatus. The bond layer also served to enhance the adhesive strengthof the coating.

The bond layer powder that was particularly effective was a Metco 480-NSgrade fully alloyed spheroidal, gas atomised Nickel 95% Aluminium 5% forwhich the data sheet indicated a particle size range of not more than 90μm and not less than 45 μm. Other commercially available bond coats andalso mixture of metals and ceramic bond coats can be used.

In particular, the applicants have found that the coating compositionsmay be usefully applied to transfer troughs, launders, ladles, skimmingtools and siphon tubes.

EXAMPLE 1

A Bond layer was applied to a prepared metal surface with a MillerThermal SG 100 Plasma Spray Torch thermal spray unit. The bond coatpowder was a Metco 480-NS grade fully alloyed spheroidal, gas atomisedNickel 95% Aluminium 5% for which the data sheet indicated a particlesize range of not more than 90 μm and not less than 45 μm. The processsettings used were as follows:—

-   -   Voltage: 33    -   Current: 650    -   Plasma Gases: Argon at 50 psi & Helium at 50 psi    -   Powder Feed Rate: 1.5 RPM at 35 psi    -   Spray Distance: 100 mm

Ceramic powder and polymer powder were mixed and subjected to a thermalspraying to form a co-deposited coating on a ladle used for transferringmolten metal to a die cavity defining the surface of a low pressuremetal die cast component. The ceramic powder was Metco 210(NS/NS-1/NS-1-G) grade zirconia stabilised by 24% magnesium oxide forwhich the data sheet indicated a particle size range of not more than 90μm and not less than 11 μm, a melting point of 2140° C. and a density of4.2 g/cm³. The polymer powder was of polymer supplied by Sulzermetcowhich had been ground to −150 +45 μm (−100 +325). The powder mixture ofMgO(24%) ZrO₂/polystyrene contained 15% volume percent (3 wt %) ofpolymer.

The co-deposition of the powder mixture was performed using a MillerThermal SG 100 Plasma Spray Torch and a Miller Thermal powder feeder,under the following settings:

-   -   Voltage: 34    -   Current: 750    -   Plasma Gases: Argon at 50 psi & Helium at 50 psi    -   Powder Feed Rate: 2.88 (rpm) at 35 psi    -   Spray Distance: 100 mm

Following co-deposition of the blended powders, the deposited coatingwas heated to 450° C. for one hour at atmospheric conditions to causethe polymer to decompose. Polymer decomposes fully at 320 to 350° C. inair. The porous, stabilised zirconia coating resulting from removal ofthe polymer by de-composition was found to comprise an excellent coatinghaving good wear resistance and adequate thermal insulation enabling itto withstand the impingement of molten metal coating also exhibited alow heat transfer coefficient, such that solidification of molten metalduring such molten metal handling operations was able to be delayeduntil molten metal had been transferred.

1. A multilayer coating for use on molten metal holding, stirring andtransfer apparatus, the coating including a bond layer applied directlyto the surface of the molten metal holding and transfer apparatus and aporous layer of ceramic material produced by co-deposition of a powderof said ceramic material and a powder of a suitable organic polymermaterial and, after the co-deposition, heating of said polymer materialto cause its removal.
 2. The multilayer coating wherein the bond layeris formed from a particulate material having at least one metalcomponent in a metallic, intermetallic, oxide, clad or alloyed form. 3.The coating of claim 2 where at least one metal component in the bondlayer is selected from the group consisting of molybdenum, nickel,aluminium, chromium, cobalt, yttrium and tungsten.
 4. The coating ofclaim 3 wherein the at least one metal component is in combination withat least one of yttria, alumina, zirconia, boron or carbon.
 5. Thecoating of claim 2 wherein the particulate material has a particle sizeof 5 to 250 μm.
 6. The coating of claim 2 wherein the particulatematerial has a particle size of 40 to 125 μm.
 7. The coating of claim 1wherein the bond layer has a thickness of 5 to 300 μm.
 8. The coating ofclaim 1 wherein the ceramic powder making up the porous layer is atleast one metal compound selected from the group of oxides, nitrides,carbides and borides.
 9. The coating of claim 1 wherein the ceramicpowder making up the porous layer is at least one metal compoundselected from the group of alumina, titania, silica, stabilizedzirconia, silicon nitride, silicon carbide and tungsten carbide.
 10. Thecoating of claim 1 wherein the ceramic powder making up the porous layeris at least one mineral compound selected from the group of ilmenite,rutile or zircon.
 11. The coating of claim 1 wherein the organic polymeris thermoplastic material selected from at least one of the group ofpolystyrene, styrene-acrylonitrile, polymethacrylates, polyesters,polyamides, polyamide-imides, and PTFE.
 12. The coating of claim 9wherein the size of the ceramic particle size is in the range of 20 μmto 400 μm.
 13. The coating of claim 9 wherein the size of the ceramicparticle size is in the range of 5-300 μm.
 14. The coating of claim 11wherein the polymer particle size is in the range of 20-400 μm.
 15. Thecoating of claim 11 wherein the polymer particle size is in the range of45-300 μm.
 16. The coating of claim 1 wherein the porous coating has athickness of from 50-600 μm.
 17. A process of providing a coating on thesurface of a metal transport and holding apparatus comprising the stepsof: applying a bond layer to the metal surface of an article; codepositing a layer of ceramic and organic polymer particulate materialonto the bond coat; and heating the layer of ceramic and organic polymermaterial to bind the ceramic material and remove the polymer material toleave a porous layer of ceramic material.
 18. The process of claim 17wherein the step of heating to bind the ceramic particles and remove thepolymer material is conducted at a temperature above the thermaldecomposition temperature of the polymer material and up to 550° C. 19.The process of claim 17 wherein the bond layer is formed of aparticulate material having at least one metal component in a metallic,intermetallic, oxide, clad or alloyed form.
 20. The process of claim 19wherein the at least one metal component in the bond layer is selectedfrom the group consisting of molybdenum, nickel, aluminium, chromium,cobalt, yttrium and tungsten.
 21. A metal holding and transfer apparatuscomprising: an article formed of a metal substrate for contacting moltenmetal, the substrate having a multilayer coating comprising an initialbond layer applied to the surface of the article, and a porousinsulating ceramic layer formed by the co deposition of powders ofceramic particles and polymer and the heating of the co deposited layerto bind the ceramic particles and remove the polymer.
 22. The apparatusof claim 21 wherein the bond layer has a thickness of 5 to 300 μm andthe porous ceramic insulating layer has a thickness of 50 to 600 μm. 23.The metal holding and transport apparatus of claim 21 wherein the bondlayer is formed of a particulate material applied to the metalsubstrate, the particular material having at last one metal component ina metallic, intermetallic, oxide, clad or alloyed form.
 24. The metalholding and transport apparatus of claim 23 wherein at least one metalcomponent in the bond layer is selected from the group consisting ofmolybdenum, nickel, aluminium, chromium, cobalt, yttrium and tungsten.